An explosive device designed to trigger a chain reaction in a planet's volcanic system, sent into the heart of Qo'noS in 2257 as Starfleet's desperate means of ending the Klingon war.
The hydro bomb is delivered by a small drone into an active volcanic shaft. Detonated there, it would cause a vast steam explosion and set off eruptions across the planet, leaving the surface uninhabitable.
It appears in the Discovery first-season finale Will You Take My Hand?. With the Federation close to defeat, Starfleet Command secretly approves a plan put forward by the Terran Philippa Georgiou, who is passed off as her dead counterpart and given command of Discovery. On Qo'noS she drops the bomb into a dormant volcano beneath an Orion settlement.
Michael Burnham and the crew refuse to accept genocide as the price of victory. Burnham persuades Georgiou to hand over the detonator, which is given to L'Rell, who uses the threat to unite the Klingon houses and end the war.